Cementitious panels have been used in the construction industry to form the interior and exterior walls of residential and/or commercial structures. The advantages of such panels include resistance to moisture compared to standard gypsum-based wallboard. However, a drawback of such conventional panels is that they do not have sufficient flexural toughness to the extent that such panels may be comparable to, if not tougher than, wood-based panels such as plywood or oriented strand board (OSB).
Building structures during their lifetimes are subjected to a variety of impact loads (e.g., hail damage, or damage from objects hurled at the buildings due to tornados or hurricanes). Not all building sheathing panels are sufficiently tough to withstand such impact loads. Where it is necessary to demonstrate impact load resistance, the sheathing panels are measured to determine the impact the panel can resist without failure.
Flexural toughness as characterized in this specification is measured as equal to the total area under the flexural load versus deflection curve for a specimen loaded in four-point bending.
Flexural toughness is measured as the total area under the load versus deflection curve for a flexural specimen loaded in four-point bending according to ASTM C947 test method.
Wood-based panels achieving significant flexural toughness usually are plywood or oriented strand board (OSB), which consist of pieces of wood that are glued together. These panels can provide flexural toughness, but each is combustible and neither is durable when exposed to water. A panel made of hydraulic cement will resist water, but is much heavier than the wood panels and has insufficient flexural toughness. It is believed that there is no panel currently available which can provide the flexural toughness of the present invention, while avoiding the deficiencies of plywood or OSB panels.
Furthermore, the need for cementitious panels configured to behave in the construction environment similar to plywood and OSB, means the panels are nailable and can be cut or worked using conventional saws and other conventional carpentry tools. It is also desirable for cementitious structural panels to have low density to facilitate handling.
The panel should be capable of being cut with the circular saws used to cut wood.
The panel should be capable of being fastened to framing with nails or screws.
The panel should be dimensionally stable when exposed to water, i.e., it should expand as little as possible, preferably less than 0.1% as measured by ASTM C 1185.
The panel should not be biodegradable or subject to attack by insects or rot.
The panel should provide a bondable substrate for exterior finish systems.
After curing for 28 days, the flexural strength of a 0.5 inch (12.7 mm) thick panel having a density of 60 lb/ft3 (961 kg/m3) to 75 lb/ft3 (1200 kg/m3)is at least 750 psi (5.2 MPa), and preferably greater than 1000 psi (6.9 MPa) as measured by the ASTM C 947 test.
It should be evident that the currently available cement-based and wood-based products and composites meet some, but not all, of the above performance characteristics. In particular, there is a need for improved cement-based panels that are lightweight having improved flexural toughness and which exceed the capability of the currently-used cement-based and wood-based by providing non-combustibility and water durability.
Although glass fibers have been used to reinforce cement, they are known to lose strength with time since the glass is attacked by the lime present in cured cement. This may be offset, to some extent, by coating the glass fibers or by using a special alkali-resistant glass. Other fibers have been suggested to reinforce cement, such as metal fibers, wood or other cellulose fibers, carbon fibers, or polymer fibers. Col. 10, lines 1-6, says, “Although they do not provide strength equivalent to glass fibers, it is possible to include some polymer fibers in the panels of the invention. Such polymer fibers, for example polypropylene, polyethylene, polyacrylonitrile and polyvinyl alcohol fibers, are less expensive than alkali-resistant glass fibers and are not subject to attack by lime.”
U.S. Pat. No. 6,241,815 to Bonen, incorporated herein by reference, discloses a composition for use in construction materials, which may be substituted for high performance concrete, patching materials, joint compounds, and the like, such as backer boards or panels, which includes a settable calcium sulfate, preferably a hemihydrate, Portland cement, a finely divided pozzolanic material, lime, and an aggregate, optionally including other additives. The volume ratio of the aggregate to the combined calcium sulfate, Portland cement, pozzolanic material, and lime (a cementitious binder) is equal to or greater than 2/1. Panels made from this composition are useful, particularly when exposed to water since they have good dimensional stability.
U.S. Pat. No. 4,199,366 A to Schaefer et al. discloses a fiber-reinforced cement-like material having short polyvinyl alcohol fibers in an amount of at least 2 volume % based on the total volume of the material. These fibers have an elongation at break of between about 4 and 8% and a modulus of more than 130 g/dtex. A process for the preparation of the material is also disclosed. U.S. Pat. No. 4,306,911 A to Gordon et al. discloses a method for the production of a fiber-reinforced hydraulically obtained setting material. U.S. Pat. No. 4,339,273 A to Meier et al. discloses a process for producing a fiber-reinforced, hydraulically setting composition, the composition produced, and the use thereof. U.S. Pat. No. 5,298,071 A to Vondran discloses a fiber-hydratable cement composition comprising a uniform dispersion interground fiber in hydratable cement powder. U.S. Pat. No. 6,528,151 B1 to Shah et al. discloses an extruded fiber reinforced cement matrix composite made by mixing cement, water, water soluble binder, and relatively short, discontinuous reinforcing fibers, preferably short polyvinyl alcohol fibers, to provide an extrudable mixture, then extruding the mixture to shape, and curing the cement. U.S. Pat. No. 6,723,162 B1 to Cheyrezy et al. discloses concrete comprising organic fibers dispersed in a cement matrix, concrete cement matrix, and premixes. Some of its examples employ polyvinyl alcohol fibers. US 2002/0019465 A1 to Li et al. discloses short fiber-reinforced cementitious composites which are self-compacting and can be prepared by adding hydrophilic polymer fibers to a cement composition containing polymeric thickener and superplasticizer. “Fracture Toughness of Microfiber Reinforced Cement Composites”, Nelson, et al., J. Mat. Civil. Eng., September/October 2002, discloses the results of fracture toughness tests conducted on thin sheet cement composites reinforced with polypropylene (PP), polyvinyl alcohol (PVA), and refined cellulose fibers under air-dry conditions. However, the cementitious products of these references have a high density. In other words, the present state-of-the-art of cement-based panels reinforced with PVA fibers as presented by these references deals with full density panels and not with the lightweight panels.
U.S. patent application Ser. No. 10/666,294, incorporated herein by reference, discloses a multi-layer process for producing structural cementitious panels (SCP's or SCP panels), and SCP's produced by such a process. After one of an initial deposition of loosely distributed, chopped fibers or a layer of slurry upon a moving web, fibers are deposited upon the slurry layer. Also, it discloses a structural cementitious panel (SCP) produced by its process, and an apparatus suitable for producing structural cementitious panels according to its process.